Mark module_extension or tag_class deprecated

[mering]

Description of the feature request:

Add a flag to mark module extensions or single tag classes deprecated and print a warning to the user when used.

Which category does this issue belong to?

Core

What underlying problem are you trying to solve with this feature?

Help users migrate away from deprecated features.

Which operating system are you running Bazel on?

No response

What is the output of bazel info release?

No response

If bazel info release returns development version or (@non-git), tell us how you built Bazel.

No response

What's the output of git remote get-url origin; git rev-parse HEAD ?

No response

Have you found anything relevant by searching the web?

No response

Any other information, logs, or outputs that you want to share?

No response

[fmeum]

While we could a deprecation attribute, couldn't you just print any kind of deprecation message you want in the module extension's implementation function? How would new functionality in Bazel allow you to do better?

[mering]

While we could a deprecation attribute, couldn't you just print any kind of deprecation message you want in the module extension's implementation function? How would new functionality in Bazel allow you to do better?

1. IIUC print() is marked as debug while deprecation is a warning.
2. Buildifier linter warns/fails for print() statement. So one always has to add a comment to disable the linter in repos where this is enforced.